There is today increased public awareness and improved scientific understanding of the hazards to health and the environment of many synthetically produced chemicals, insecticides, herbicides and other toxic materials. Of particular concern due to their high toxicity and persistence are halogenated organic compounds such as PCBs, Dioxin, DDT, monochlorobenzene, dichlorophenol, pentachlorphenol, Dieldrin, Aldrin, 2,4-D, 2,4,5-T, and other compounds such as Paraquat, Diquat, Phorate, Bromicide, carbamates and Atrazine. There is therefore a need for effective methods of disposing of such toxic materials. The wide spread use of PCBs as dielectric fluid additives in transformers and other electrical equipment, due to their excellent insulating properties, present a particularly serious disposal problem.
In some countries stockpiles of chemical weapons, which include organophosphorus nerve agents and mustards, await a suitable means of disposal. This disposal is required under the terms of the Chemical Weapons Convention of 1993. The disposal of such materials presents a particularly severe problem, since accidental dispersal could result in enormous loss of life. A disposal system with extremely low risk factors is required for this application. A deadline for weapon destruction of Dec. 31, 2004 has been set by the Chemical Weapons Convention.
Currently proposed methods of disposing of toxic materials typically involve high temperature incineration, biochemical or chemical treatment. High temperature incineration seeks to destroy toxic waste materials by converting them to gaseous products. Any toxic gases, such as hydrogen chloride, must then be removed from the effluent gas before it is released to the atmosphere. If operating conditions are not closely controlled, there is a possibility that toxic materials will be released into the environment either through incomplete destruction of the original materials or through creation of new materials in the incinerator or through inefficient gas cleaning. Control systems are required to regulate the fuel addition, air flow, temperature, flame, gas composition, scrubbing liquor flow and so on. Back-up systems to deal with loss of electrical power are also required. These systems comprise a very large number of individual components, both electrical and mechanical, and the failure of any one of them may lead to the immediate loss of integrity of the system as a whole. Failure of the system could lead to widespread dissemination of toxic material into the surrounding environment.
Chemical treatment results in chemical decomposition of the toxic materials through the action of suitable reagent mixtures. U.S. Pat. No. 5,064,526 to Rogers et al discloses a method for both the decomposition and removal of halogenated and non-halogenated organic compounds contained in a contaminated medium by the use of an alkali or alkaline earth carbonate or bicarbonate or hydroxide, a hydrogen donor such as an oil and a catalytic form of carbon such as a carbohydrate. This process is conducted at elevated temperatures, requiring the application of heating and cooling systems, fire prevention systems, power failure systems and gas emission systems. These systems comprise a multiplicity of components and interconnections, each of which is the subject of possible failure, and the malfunction of anyone of them may lead to a loss of the integrity of the system as a whole. In the event of a failure leading to a fire, there is potential for widespread dissemination of toxic material into the surrounding environment.
Processes involving the use of baths of molten metal or salt, plasma arcs or other operating conditions departing significantly from ambient suffer from the common disadvantage of requiring complex systems to ensure containment, maintenance of optimum operating conditions, and prevention of emissions. Risk of failure tends to increase as the number of overall system components and interconnections increases and the potential for catastrophic emission of toxic materials tends to increase with departure from ambient conditions.
From a public, safety and commercial liability viewpoint, the acceptability of any process for toxic waste disposal is largely dependent on the risk of system failure and the potential consequences of such failure. The high risk potential of the proposed processes referred to above limits their acceptability and thus limits their utility.
Furthermore, the perceived risks associated with incineration have resulted in such widespread public opposition that any process resembling incineration in any way is liable to be rejected on the basis of its similarity rather than on a scientific assessment.
Biological methods of disposal of toxic materials do not suffer from most of the above-mentioned disadvantages, but such methods are not able to treat concentrated forms of toxic materials directly.
Toxic waste materials may not be well characterised and may contain mixtures of organic and inorganic materials, and the toxic materials may be contained in corroded drums or within electrical components. It is desirable therefore that a process be capable of disposing of a wide range of materials and containers in a single stage, thus eliminating the risks associated with having a number of separate handling stages. Many of the processes proposed to date are not capable of handling toxic organic compounds when they are mixed with inorganic materials such as arsenic trioxide, nor are they capable of accepting the containers holding the toxic wastes.
The process of the invention is based on the discovery that mechanical activation can induce chemical reactions which break down the molecular structure of toxic materials and form products which are simple, non-toxic compounds. It was previously not known to use mechanical activation for the destruction of toxic materials, nor was it known that complex organic molecules could be completely destroyed by mechanical activation.
Mechanical activation involves the use of mechanical energy to increase the chemical reactivity of a system so as to induce mechanochemical reactions which involve changes in chemical composition as a consequence of the applied mechanical energy. For example, one form of mechanical activation is mechanical alloying by which alloys are formed from pure starting materials by milling the constituents in a high energy ball mill. During milling the energy imparted to the reactants through ball/reactant collision events causes the starting materials to react, enabling the formation of an alloy without the need for melting or high temperatures. Another form of mechanical activation, described in International Application No. PCT/AU89/00550, is concerned with a chemical reduction process involving mechanically activated chemical reduction of reducible metal compounds for manufacturing metals, alloys or ceramic materials.
The use of mechanical activation to synthesise certain types of chemical compounds, such as organometallic compounds, is described in U.S. Pat. No. 2,416,717 by Shaw. An example of the kind of reaction described by Shaw is the so-called Grignard type of reaction which is used to synthesise more complex organic compounds from simpler compounds. When used to carry out a Grignard type of reaction, mechanical activation by continuously cutting chips from a metal used to make a Grignard type reagent may improve the reactivity of the reagents and may be used to control and regulate the rate at which the chemical reaction proceeds. However it was not appreciated that mechanical activation could also be used to break down complex organic compounds into simple inorganic substances.
Mechanochemical degradation of polyvinyl chloride (PVC) during mechanical grinding in a vibrational mill has also been investigated. PVC composites which contain inorganic fillers have been subjected to grinding to help characterise the effect of the filler on the stability of the PVC composite. The degree of polymerisation and dehydrochlorination of the PVC was found to vary with the addition of calcium compounds such as CaSO.sub.4.2H.sub.2 O, CaCO.sub.3 and Ca(OH).sub.2. However this research into the effects of mechanical grinding on a polymer powder (PVC) did not anticipate or in any way consider the use of mechanical activation for the destruction of toxic materials such as halogenated organic compounds into simple inorganic compounds such as carbon.